Question at position 38 What is the primary mechanism by which urine is expelled from the bladder?Contraction of detrusor muscleContraction of the external urethral sphincterRelaxation of the external urethral sphincterContraction of the internal urethral sphincterRelaxation of the detrusor muscle

Question

BlackTom AI · Accepted Answer

The question asks about the primary mechanism by which urine is expelled from the bladder.

Option 1: Contraction of detrusor muscle. This is the muc-smooth muscle layer of the bladder wall that contracts during micturition, generating the pressure needed to push urine out through the urethra. This is the central driver of voiding.

Option 2: Contraction of the external urethral sphincter. The external sphincter is a voluntary skeletal muscle that normally remains contracted to retain urine; during urination it must relax. Therefore, its contraction would oppose urine expulsion rather than cause it.

Option 3: Relaxation of the external urethral sphincter. Relaxation of this sphincter is necessary for urine to exit once the detrusor contracts, because a closed sphincter would block flow. However, relaxation alone does not generate the pushing force; it permissively enables voiding. The primary driving force remains detrusor contraction.

Option 4: Contraction of the internal urethral sphincter. The internal sphincter (a smooth muscle) remains closed to retain urine in the bladder and must relax during micturition. Contraction would impede urination, making this option incorrect as the primary mechanism.

Option 5: Relaxation of the detrusor muscle. Relaxing the detrusor would stop the bladder from generating pressure and would prevent, not cause, urine expulsion. Hence, this option is fundamentally inconsistent with the act of urination.

In summary, the critical event that produces the force required to expel urine is the contraction of the detrusor muscle, while other options describe actions that either oppose voiding or are secondary permissive steps rather than the primary driving mechanism.

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